Microelectromechanical systems (MEMS) refers to a technology that integrates micromechanical structures (referred to hereinafter as microstructures) and microelectronic circuits on the same substrate to create an integrated device. MEMS devices are utilized in, for example, pressure sensing, acceleration sensing, inertial sensing, switches, motors, and the like. While the microelectronic circuits are fabricated using integrated circuit (IC) process sequences (e.g., CMOS, Bipolar, or BICMOS processes), the microstructures are fabricated using compatible “micromachining” processes.
The choice of materials and fabrication processes for implementing MEMS technology depends on the device being created and the market sector in which it will operate. Typical micromachining processes that may be employed for fabricating the microstructures include, for example, surface micromachining and bulk micromachining. In surface micromachining, the MEMS device is fabricated by depositing a sacrificial layer onto a substrate. A layer of polysilicon, as the structural micromechanical material, is then deposited over the sacrificial layer and is etched to yield a desired shape for the particular microstructures. The layer of sacrificial material underlying the polysilicon may then be etched to open up passageways or clearances between moving parts of the microstructures. Thus, surface micromachining is based on the deposition and etching of different structural layers on top of the substrate. In contrast, bulk micromachining defines structures by selectively etching directly into a silicon wafer to produce the mechanical microstructures from the single crystal silicon itself.
Conventional MEMS capacitive sensors operate so that a flexibly mounted seismic mass, also known as a proof mass, is deflectable in at least one direction by a property being sensed, e.g., acceleration. Deflection of the proof mass causes a change in capacitance of a differential circuit that is connected to it. This change in capacitance is a measure of the property being sensed. The aspect ratio of a mechanical microstructure is the ratio of its height relative to its lateral width. A high aspect ratio microstructure can advantageously provide the benefits of increased sense capacitance and reduced cross-axis sensitivity in a MEMS capacitive sensing device.
Bulk micromachining processes can be used to produce these high aspect ratio microstructures. However, bulk micromachining tends to be more limited and more costly than surface micromachining.
In surface micromachining, the use of polysilicon build up layers increases the design freedom for integration of complicated, movable microstructure features. Design freedom includes many more possible layers that can be achieved, relative to bulk micromachining. However, the polysilicon build up layers can be limited in thickness due to residual stress, thus making the thin film layers flexible out of the plane of fabrication. Stress can cause cracks, de-lamination, and voids. In addition, stress results in mismatch of coefficient of thermal expansion and non-uniform deformation. Thus, stress can decrease the longevity of a component and can cause malfunctioning during normal operation. Accordingly, it is difficult to achieve a desired high aspect ratio using a surface micromachining process.
As micromachined devices increase in complexity it becomes increasingly important to improve their electrical flexibility. One approach to improving electrical flexibility is to provide electrical isolation between the various microstructure elements that are still mechanically one piece and to provide electrical isolation from the microelectronic circuits in order to enhance the performance of the MEMS device. In bulk micromachining, electrical isolation has been accomplished by separating conducting metal layers by insulating dielectric layers and through the implementation of trench isolation structures separating laterally adjacent microstructures. However, electrical isolation techniques implemented in bulk micromachining processes cannot be readily implemented in a surface micromachining process.